In general, one means for enhancing the strength of sintered bodies involves the addition of a predetermined amount of impurities to the base material, whereby the growth of grains during sintering is inhibited. Another means for strengthening sintered bodies involves the coating of particles on the surface of the formed base material mass, said particles having an ion radius larger than that of said base material mass, thereby to cause substitution and solid-solution formation for producing compression stress in the surface of said base material mass.
According to the reinforcing manner in which the aforesaid impurities are added, however, the strength of the base material may be affected depending upon the type of said impurities. This requires time-consuming examination of the type of impurities. Thus, such a manner for reinforcing sintered bodies cannot be said to be a generally applicable reinforcing treatment without difficulty. According to another reinforcing manner to form the coating layers, the compression stress varies depending upon the type of the coating materials, viz, the degree of substitution and solid solution formation with respect to the base material, with resulting variations in strength. This leads to the deficiency of reinforced stability and, in some cases, may result in the formation of cracks on the surface of sintered bodies.